This invention relates to foundry techniques used to create sand cast metal parts. More particularly, the present invention relates to a new and improved sand casting foundry composition and method using shale as an anti-veining agent to prevent veining defects in the cast metal parts.
Sand casting is a process used in the foundry industry to produce cast parts. In sand casting, disposable foundry shapes are made by forming a sand-based foundry composition into predetermined configurations and curing the composition to preserve those foundry shapes. A binder in the foundry composition maintains the predetermined configuration of the foundry shape. The foundry shape which defines the exterior of the resulting cast part, known as a mold, is positioned relative to the foundry shape which defines the interior of the cast part, known as a core. With the mold and the core foundry shapes oriented as desired, molten metal is poured between them. The foundry shapes confine the molten metal while it cools and solidifies into the resulting cast part.
The binder must have the capability to preserve the predetermined configurations of the mold and core foundry shapes while those foundry shapes are oriented in the appropriate relationship to create the cast parts and during the time while the molten metal solidifies into the cast part. The typical type of foundry sand used for this purpose is silica sand, although other useful foundry sands include chromite, zircon and olivine sands. Two basic types of binders are commonly employed: inorganic binders, such as clay; and chemical binders, such as phenolic resin binders.
The most widely used inorganic binder for a sand-based foundry composition is bentonite clay. The foundry composition of the sand and bentonite clay binder is referred to as green sand. Green sand is a water tempered sand mixture having plasticity. A green sand foundry composition is typically formed by mulling silica sand, bentonite and a small amount of tempering water. The tempering water allows the bentonite to become sufficiently plastic so that it may be smeared relatively uniformly and thinly over the sand grains during the mulling process. The thin coating of the bentonite on each sand grain interacts with the thin coating on the adjacent sand grains causing the sand grains to be held in place in the mold and core foundry shapes. Green sand molding is economical and is widely used to cast ferrous as well as non-ferrous metal parts. Green sand molding permits high quantity, high quality foundry production, particularly for smaller cast parts.
Chemically-bonded, sand-based foundry compositions use a variety of polymerizable or curable organic and inorganic resin binders to hold the sand grains together in the desired mold or core shape. Chemical bonding involves mixing the sand and a polymerizable or curable binder. Once the mixture of the sand grains and the uncured binder have been shaped into the desired configuration, the chemical binder is polymerized or cured by the addition of a catalyst and/or heat, resulting in converting the shaped configuration into hard, solid, cured mold or core foundry shapes. Examples of curable resin binders include phenolic and furan resins. In a typical no-bake process, i.e. one which does not involve the addition of heat for curing, the sand, binder, and a liquid curing catalyst are mixed and compacted to produce the desired configurations of the mold or core foundry shapes. A commonly used no-bake binder is a polyurethane binder, derived by curing a polyurethane forming binder material with a liquid tertiary amine catalyst.
When subjected to the heat of the molten metal, the sand grains in mold and core foundry shapes expand. If the sand grains in the molds and cores are too close together, the sand grains expand in size and push on the adjacent sand grains. The thermal expansion opens up small cracks and fissures in the molds and cores, and the molten metal penetrates into those cracks and fissures. When the molten metal solidifies, raised, narrow ridges on the surfaces of the cast part result at those locations where the molten metal penetrated into the small cracks and fissures. The resulting narrow ridges are referred to as xe2x80x9cveinsxe2x80x9d or xe2x80x9cveining.xe2x80x9d The veining may make it necessary to surface grind or machine away the projecting veins. Of course, such surface grinding or machining increases the cost of producing the cast part.
Another type of foundry shape defect is caused by gas formation, particularly within core foundry shapes. Water in green sand casting foundry compositions will volatilize into steam in the presence of the hot molten metal. Trapped steam may cause pin holes or cracks in the foundry shape, resulting in the metal penetration into the foundry shape. The gas may also create an uneven or discontinuous surface in the cast part. Gas pressure also results from the volatilization of certain chemical constituents in foundry compositions. It is desirable to use chemical binders which are not susceptible to excessive volatilization, particularly in core foundry shapes.
Expansion and cracking from gas pressure is more of a problem in core foundry shapes, because core foundry shapes are typically surrounded by the molten metal due to their internal position. Those binders which produce significant amounts of gas when exposed to metallurgical temperatures may only be used in foundry shapes where the confined gas has an avenue to escape, otherwise the gas itself may induce cracks, fissures and pin holes. Mold foundry shapes are exposed to the ambient atmosphere and therefore provide avenues for the gas pressure to escape, although the gas pressure may nevertheless create defects in mold foundry shapes. To avoid excessive gas creation where a clay binders is used, the amount of tempering water used to activate the clay binders and allow it to be smeared over the sand grains is limited.
A wide variety of different agents have been added to sand casting foundry compositions in an attempt to improve the properties of core and mold foundry shapes to avoid veining and other casting defects. These additives, known generically as anti-veining agents, include starch based products, dextrin, fine ground glass particles, red talc and wood flour, i.e. particles of wood coated with a resin, granular slag, pulverized sea-coal, alkaline earth or alkaline metal fluoride, and lithia-containing materials, among many other things. Iron oxide, including red iron oxide, also known as hematite (Fe2O3), black iron oxide, also known as magnetite (Fe3O4), yellow ochre, and Sierra Leone concentrate, is also another widely used antiveining agent.
Each of these anti-veining agents are theorized to function in a different way to avoid or reduce the incidence of cracks, fissures and the other defects in the foundry shapes which cause veining. It is generally believed that the iron oxides increase the hot plasticity of the sand mixture by the formation of a glassy layer between the sand grains. The glassy layer deforms without fracturing at metallurgical temperatures, to prevent fissures in the foundry shapes. Grains of slag are thought to become soft at metallurgical temperatures permitting the sand grains to expand. Sea-coal and other combustible anti-veining agents are believed to form volatile gas at metallurgical temperatures leaving void space into which the sand grains expand.
The present invention relates to the use of particles of shale as an anti-veining agent in a sand casting foundry composition used to create foundry shapes for casting metal parts. Phyllosilicate mineral components of the shale particles will undergo crystal structural collapse when the foundry shape is heated by the molten metal during casting. With a sufficient concentration or volumetric quantity of shale particles distributed within the foundry shape, and with sufficient sizes of the shale particles, the collapse of the crystal structure of the phyllosilicate mineral components of each shale particle will cause the shale particles to yield space within the foundry shape sufficient to compensate for the thermally-induced physical expansion of the sand grains. The net result is a negligible change in volume of the foundry shape during heating, thereby avoiding the mechanical forces which cause cracks and fissures in the foundry shape that result in veining.
The volumetric quantity of the shale particles necessary to yield the physical volume sufficient to compensate for the physical expansion of the sand grains may be achieved by using a relatively larger number of relatively smaller physically-sized particles or a relatively smaller number of relatively larger physically-sized particles in the foundry composition. An advantage of using a relatively smaller number of relatively larger sized particles is that less resin binder is consumed by the shale particles. Resin binder is added to and mixed with the mixture of the sand grains and shale particles to form the foundry composition. Since resin binder is expensive, it is important to limit the quantity used to the smallest amount necessary to achieve adequate tensile strength of the foundry shapes to resist breakage or deformation when the foundry shapes are positioned to cast the metal part and while confining the molten metal as its solidifies into the cast part. More surface area is exhibited by a larger number of smaller sized particles as compared to a smaller number of larger sized particles which occupy the same volumetric space. The amount of resin binder consumed is directly related to the surface area of the particles which must be coated with that resin binder, and it is for this reason that a relatively fewer number of relatively larger sized shale particles is preferred.
The shale particles include accessory mineral components which are distributed throughout each particle and are interspersed with the collapsible crystal phyllosilicate mineral components. The accessory mineral components are preferably harder than the collapsible crystal phyllosilicate mineral components in the shale particles. The preferred harder accessory mineral components give the shale particles structural strength and durability to substantially resist disintegration into smaller particles when the shale particles are mixed with the sand grains to form the foundry composition. Consequently, using the preferred durable shale particles in the foundry composition preserves the advantage of using less resin binder in the foundry composition.
The accessory mineral components within the preferred type of shale particles also substantially reduce the porosity and increase the impermeability of the shale particles within the foundry composition. The preferred reduced porosity and increased impermeability also makes the shale particles less sorptive of resin binder. The preferred harder and more durable accessory mineral components of the shale particles also avoid creating significant reductions in tensile strength of the foundry shape when bound by the resin within the matrix of sand grains compared to other currently used anti-veining agents.
These and other improvements are obtained in a number of different forms of the present invention. A sand casting foundry composition reduces thermal defects that cause veining in metal parts cast from sand casting foundry shapes formed from the foundry composition. Such a foundry composition comprises a plurality of foundry sand grains, a plurality of shale particles substantially uniformly distributed throughout the sand grains to form a matrix of sand grains and shale particles, and a curable binder coating the sand grains and the shale particles to hold sand grains and shale particles in position within the matrix upon curing. A method of making the foundry composition involves mixing a plurality of foundry sand grains with a plurality of shale particles to form a mixture in which the shale particles are substantially uniformly distributed within the sand grains in the mixture, and coating the mixture of sand grains and the shale particles with a binder sufficient to hold sand grains and shale particles in position relative to one another after curing of the binder. A method of making a foundry shape from the foundry composition involves mixing a plurality of foundry sand grains with a plurality of shale particles to form a mixture in which the shale particles are substantially uniformly distributed among the sand grains in the mixture, coating the mixture of sand grains and shale particles with a binder sufficient to hold sand grains and shale particles in place relative to one another after curing of the binder, shaping the binder-coated mixture into a predetermined configuration of the foundry shape, and curing the binder while maintaining the predetermined configuration. A method of casting a metal part using core and mold foundry shapes formed in this manner involves positioning the core and mold foundry shapes relative to one another to define the metal part to be cast, pouring molten metal in the space between the core and mold foundry shapes, and solidifying the molten metal while confined between the core and mold foundry shapes.
These aspects of the invention may also be supplemented by further preferable improvements. The shale particles are selected to have a phyllosilicate mineral component with a crystal structure which inherently collapses upon exposure to the temperature created by molten metal used in casting the metal part. Upon collapse, volumetric space is yielded by the shale particles to compensate for and counterbalance the additional volumetric space consumed by the thermal expansion of the sand grains, thereby avoiding the creation of mechanical forces and stresses within the foundry shape that lead to veining. The crystal structural collapse is preferably achieved by phyllosilicate mineral components of the shale particle which have a crystal structure that collapses in response to the elevated temperatures caused by the molten metal. The volumetric concentration and size of shale particles determines the desired yield volume to compensate for the thermal expansion of the sand. The binder is preferably added after the sand grains and shale particles have been mixed, thereby facilitating the homogenous distribution of the shale particles among the sand grains while evenly coating the sand grains and shale particles sufficiently to hold them together in the predetermined desired foundry shape. This sequence of addition will use less binder that if the binder is added before the sand grains and the shale particles have been mixed together. There are many other desirable improvements described herein which may be practiced with the different aspects of the present invention.
A more complete appreciation of the scope of the present invention and the manner in which it achieves the above-noted and other improvements can be obtained by reference to the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings, which are briefly summarized below, and by reference to the appended claims.